1. Field of the Invention
The present invention relates to a structure of a wavelength tunable semiconductor laser, and more specifically to a semiconductor laser capable of tuning a wavelength by integrating a sampled grating distributed feedback Bragg (SG-DFB) structure and a sampled grating distributed Bragg reflector (SG-DBR) structure.
2. Description of the Prior Art
Recently, the text data basis is moved into the multi-media data basis in a communication contents with wide use of Internet, and thus the data transmission rate and capacity required are largely increased. Furthermore, as one method of expanding a transmission bandwidth, a method of transmitting an optical signal using a wavelength division manner is used. This method is to concurrently send the different informations having different wavelengths through an optical fiber, thereby largely expanding the bandwidth of a single optical fiber. In addition, according to this method, it is possible to reduce cost for installing the optical fiber and to implement a more flexible and more expandable optical network. Therefore, this method will be an essential transmission method in the future of an optical communication network.
In such a WDM optical communication system, when a wavelength tunable semiconductor laser diode instead of a fixed wavelength semiconductor laser diode is used as a light source, there are several advantages in the system. Specifically, it is possible to reduce the number of the light source for back-up for maintaining the system and simplify the network control software as well as dynamically provide a wavelength. In addition, as a dense WDM (that is, the DWDM), in which the interval between wavelengths of the WDM optical communication system is 0.8 nm or 0.4 nm, is gradually developed, the wavelength tunable laser diode has several economical advantages in comparison with single wavelength laser diodes for generating fixed wavelengths, respectively.
As representative wavelength tunable laser diode having been proposed up to now, there are a sampled grating distributed Bragg reflector (SG-DBR) laser diode, a super-structure grating distributed Bragg reflector (SSG-DBR) laser diode, a grating-assisted codirectional-coupler with sampled grating reflector (GCSR) laser diode, and so on.
Now, the conventional wavelength tunable semiconductor laser will be described with reference to the appended drawings.
FIG. 1 is a constructional view of a sampled grating distributed Bragg reflector (SG-DBR) laser diode disclosed in U.S. Pat. No. 4,896,325.
The wavelength tunable laser diode shown in FIG. 1 comprises total four areas, that is, SG-DBR areas 140, 142 at both sides of the wavelength tunable laser diode, and a gain area 136 and a phase control area 132 in which an optical wave is generated. Furthermore, in order to tune a wavelength of such a SG-DBR laser diode, a Vernier control circuit 148 for continuously tuning a wavelength, an offset control circuit 150 for discontinuously tuning a wavelength, a phase control circuit 146 in a phase area, and a gain control circuit 144 are needed.
As described above, according to the fundamental operation principle of such a laser diode, it is possible that the optical wave distributed over a wide wavelength range is oscillated at a special wavelength band by making only the optical wave with the special wavelength band resonate. In other words, the SG-DBR areas 140, 42 are integrated at both sides of the gain area so that only the selected special wavelength band resonates to be tuned.
The SG-DBR areas 140, 142 have the sampled diffraction lection grating construction as shown in FIG. 2 and the reflected spectrum characteristics as shown in FIG. 3. A central peak of the reflected spectrum is a Bragg wavelength λB determined by a diffraction grating pitch (period) λ, and an interval between the peaks is determined by a period Z of the sampled grating (SG). In other words, the laser diode can be oscillated at the matched peak of the several peaks in both sides by integrating the SG-DBR areas having the different SG periods Z from one another in the both sides of the laser diode.
Furthermore, it is possible to vary the refraction index in the SG-DBR area to vary the matched peak, so that the oscillation wavelength can be tuned. The phase control area 132 adjusts an interval between longitudinal modes of the gain area 136 generated by the SG-DBR to continuously tune the wavelength or match the longitudinal modes with the reflection peak, so that the power of the oscillation wavelength is maximized. According to this principle, it is possible to appropriately adjust the refraction index of the SG-DBR areas 140, 142 in both sides of the laser diode and the phase control area 132 using a current, so that the wavelength can be continuously or discontinuously tune.
However, in order to tune the wavelength of such a SG-DBR laser diode, since various control circuits such as a Vernier control circuit 148, an offset circuit 150 for shifting a discontinuous wavelength, a phase control circuit 146 in a phase area, a gain control circuit 144, etc. are needed, there is a problem that a laser diode module and a system circuit are complex.
In addition, there is a constructional problem that the optical output efficiency is reduced due to the optical loss generated in the SG-DBR areas 140, 142 of the both sides of the laser diode. In order to overcome these problems, investigations for incorporating semiconductor optical amplifier (SOA) into the SG-DBR laser diode have been actively progressed. However, there is a problem that the construction of the laser diode is more complex, so that it is difficult to produce the laser diode.
On the other hands, a SSG-DBR laser diode disclosed in U.S. Pat. No. 5,325,392 will be described with reference to FIGS. 4 and 5.
The SSG-DBR laser diode disclosed in the U.S. Pat. No. 5,325,392 comprises SSG-DBR areas at both sides for tuning the wavelength, a gain area and a phase control area, similarly to the SG-DBR laser diode shown in FIG. 1. The diffraction grating in the SSG-DBR laser diode is constructed to have a construction obtained by modulating a diffraction grating period repeatedly at the special period Z as shown in FIG. 4. Therefore, according to this construction, the reflected spectrum has the interval between reflection peaks determined by the period Z as shown in FIG. 5 and each reflection peak has a large and constant value even with a small coupling coefficient due to modulation of the period.
Such a SSG-DBR laser diode has a wide wavelength tunable area and a constant power in accordance with the tuned wavelength. However, the SSG-DBR laser diode has a constructional limit due to the SSG-DBR areas at the both sides similarly to the SG-DBR laser diode, so that it is difficult to produce the SSG-DBR laser diode due to the complex structure of the diffraction grating.
As a conventional wavelength tunable laser diode other than the aforementioned wavelength tunable laser diode, there are a GCSR laser diode, a wavelength tunable twin-guide laser diode, and so on. However, in such laser diodes, since a re-growth process and an etching process must be repeated to produce the laser diodes, there is a problem that it is difficult to produce the laser diodes and thus the laser diodes are it is not suitable for mass production.
In brief, the conventional wavelength tunable laser diode has problems that the structure thereof is complex, the output optical efficiency is low, and the wavelength tunable control is complex.